One aspect of the present invention is directed to the treatment of an absorption liquid, such as a concentrated salt solution and the like which may be used in the dehumidification of gas such as air. During such dehumidification process the concentrated salt solution is being diluted by absorbing moisture from the air. The diluted salt solution is reconcentrated prior to being reintroduced into the absorption process.
Another aspect of the present invention is directed to concentrating spent liquor from pulping processes in evaporating plants in which the waste liquor is concentrated to a degree of dryness which permits the combustion thereof to recover the pulping chemicals. In conventional evaporating plants the liquor is generally concentrated by evaporation in one or more separate evaporation stages. The vapor from the last stage is usually condensed by bringing the vapor into indirect contact with cooling water in a condenser as, for example, the one disclosed in U.K. Patent Application No. GB 2 000 584.
The present invention is directed to a method and apparatus for concentrating a liquid by evaporation resulting in high thermal efficiency by utilizing the heat energy of vapor and preferably by utilizing the heat energy of the vapor leaving the evaporator instead of removing heat energy from the evaporating system to an external cooling water system. It will, however, be understood that the heat required for the evaporation of liquid may be derived from sources other than the vapor exiting from the evaporator. The present invention can thus be applied to the evaporative concentration of liquids in general and the specific examples given herein should thus not be construed to limit the scope of the present invention in any manner.
According to one embodiment of the invention, air is dehumidified by direct contact with a water-absorbing liquid. An aqueous solution of an easily soluble salt such as potassium acetate, sodium acetate, potassium carbonate, calcium chloride, lithium chloride and lithium bromide or the like or mixtures thereof is used as the absorption liquid. These concentrated salt solutions exhibit great affinity to water. Consequently, the water vapor pressure above the solution is correspondingly low.
If air at a certain temperature and of a certain relative humidity is brought into contact with such a concentrated salt solution, water vapor from the air is absorbed by the solution as long as the water vapor pressure above the salt solution is lower than that reached during the state of equilibrium.
When air is dehumidified by absorption of water vapor, the absorption liquid will become increasingly diluted by the absorbed water. As the only volatile component of the absorption liquid is water, the absorption liquid can be regenerated by evaporation. This is usually accomplished by heating the absorption liquid to a temperature at which the water vapor pressure thereof exceeds the atmospheric pressure thus causing the water to evaporate. The boiling point elevation of the concentrated salt-water solution suitable for absorption purposes is high. Generally, the dilution of the absorption liquid by absorption of water vapor is relatively small and, consequently, evaporation in more than one stage is usually not feasible so that the diluted absorption liquid is usually regenerated by evaporation in a single stage evaporator.
To regenerate the absorption liquid in an evaporator an amount of energy corresponding to the heat of vaporization is required. Additional energy is needed to heat up the liquid to the boiling temperature thereof and to compensate for heat losses and the like.
According to a preferred embodiment of the present invention, the heat energy of the vapor leaving the evaporator is used to evaporate water from the absorption liquid. This is accomplished by bringing the vapor into indirect contact with the absorption liquid either before or after the absorption liquid is concentrated in the evaporator or both. The vapor exiting from the evaporator is brought into contact with one surface of a heat exchange element while the absorption liquid is brought into contact with the other surface of the heat exchange element. The absorption liquid is preferably caused to flow down in form of a thin film on the surface of the heat exchange element. In addition, air is caused to flow in contact with the absorption liquid to lower the water vapor pressure above the absorption liquid thus enhancing the evaporation of water from the absorption liquid. The air will be saturated by water vapor and the vaporization heat is removed from the surface of the heat exchange element.
By condensing the vapor from the evaporator by means of the absorption liquid which thus will be concentrated before the evaporator and/or after the evaporator, a higher coefficient of performance as well as significant energy savings are obtained. An additional advantage of the invention is that the need of an external cooling water system is eliminated. The method and apparatus of the present invention thus provide a process similar to an evaporation in two stages or two effects with its considerably lower specific energy consumption. Although only one evaporator is shown in the accompanying drawings, it will be understood that more than one evaporator can be used. The preferred process of the present invention, i.e. when the vapor from the last evaporator stage is utilized for concentrating the solids containing liquid such a the absorption liquid or the spent liquor, results in an evaporation of n+1 effects, whereby n can be one or an integer greater than one, and is preferably below about ten. For example in a system with three evaporators in series (i.e. n equals three) the process of the present invention effectively results in evaporation corresponding to four stages or effects.
This and other objects of the present invention will become clear from an inspection of the drawing, the detailed description of the invention and from the appended claims.